It probably means 3 campaigns like in GTAIV. You go through 1 story in 3 different perspectives.

Actually as far as I know it's all simultaneous and you can swap between any 3 at any time, especially during heist missions where each 3 has their own role in the plan. You take control of one then swap to another and the game takes control of the other 2 who aren't under your control.

The mechanics are the first thing you notice, but it’s the pacing that makes the stronger impression. The constant character switching ensures the action doesn’t let up until the shooting stops, while regenerating health, another series first, encourages ballsy play. The trio of protagonists doesn’t solely mean Rockstar gets to tell three stories at once; there are genuine gameplay benefits to the system, too.

Not sure how any of that is relevant or new information.There is still no actual GTA V released for the PC right now, and the fact that they have done this before is again completely pointless to point out.